Method and apparatus for receiving diversity signals for use in OFDM radio communication system

ABSTRACT

A diversity receiving apparatus for use in an OFDM transmission radio communication/broadcasting system where OFDM signals are received by a plurality of antennas and input into a plurality of receivers. The OFDM received signals are further transformed into frequency spectra. Channel frequency responses are calculated by using OFDM received signal frequency spectra and a reference frequency spectrum generated in a reference spectrum generator. OFDM received signal frequency spectra distortions are compensated by using corresponding channel frequency responses. The transformed OFDM received signal frequency spectra are input into a selector unit as first input signals, while the distortion-compensated OFDM received signal frequency spectra are input into the selector unit as second input signals. The selector unit then selects a second input signal corresponding to a first input signal having maximum amplitude or maximum power. A demodulator then demodulates the output signal of the selector unit into a digital signal sequence.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to an orthogonal frequencydivision multiplexing (OFDM) receiving apparatus for use in a basestation or a terminal station of a radio communication system thatperforms OFDM radio transmission, or to an OFDM receiving apparatus foruse in a receiving station of an OFDM broadcasting system. Moreparticularly, the invention relates to an OFDM diversity receivingapparatus that implements high-quality information transmission evenunder a poor radio propagation environment.

2. Description of the Background

Terrestrial television broadcasting currently use analog signals. Thereare plans, however, in about the year 2000, for analog signals to bereplaced by digital signals in terrestrial television broadcasting. Interrestrial television broadcasting, the area covered by a singletransmission station is very large. A serious problem thus encounteredis that multiple reflection wave transmission (multipath) occurs inpropagating radio waves, thus degrading image quality. This is referredto as “ghost interference”. The scale of the multipath in terrestrialtelevision broadcasting is much greater than that occurring in radiocommunication system, such as mobile cellular telephones. Even using anadaptive equalizer, which should be effective as a countermeasureagainst multipath, the multipath in terrestrial television broadcastingcan no longer be handled.

In view of the above background, the OFDM transmission method, which is,in principle, resistant even under a poor multipath transmissionenvironment and achieves high-quality information transmission, is to beemployed as a digital transmission method for terrestrial televisionbroadcasting. An OFDM transmission signal is provided with a guardinterval in which part of a transmission waveform (symbol) is copied.The guard interval accommodates multipath propagation which is shorterthan the length of the guard interval, thereby preventing fataldegradation of the image quality.

There are high expectations that the above-described multipath-resistanttransmission characteristics of the OFDM transmission method will beused not only in terrestrial television broadcasting, but also inwide-area radio communication systems, such as multimediacommunications, which are to be widely performed in public networks.Accordingly, a technical study of the practical application of the aboveOFDM transmission method is actively in progress.

Additionally, according to the OFDM transmission method, by virtue ofthe multipath-resistant transmission characteristics, a single frequencynetwork (SFN) in which the same information is simultaneouslytransmitted with the same frequency can be constructed. This makes itpossible to prevent wasteful use of a frequency, and more specifically,to prevent the transmitting frequency from being changed according tothe area, which is conventionally required. In terms of compressing afrequency band for use, the OFDM transmission method is very effective.

However, even though the OFDM transmission method exhibits excellentmultipath-resistant transmission characteristics, the receivingcharacteristics cannot be perfectly maintained because of severefrequency selective fading caused by multipath propagation, or Dopplershift and time fading occurring while a receiving end is moving.

High-definition image transmission is strongly expected to be achievedin digital terrestrial television broadcasting and the next-generationmultimedia communications. Particularly in this type of transmission,there is a demand for a radio transmission technique which is much morestable and which exhibits higher quality transmission than that beingused in current mobile cellular telephones, in which soundcommunications rather than image communications are the mainstream.Accordingly, the advent of a receiving method and a receiving apparatusthat exhibit higher-quality receiving characteristics is greatly needed.

High-definition images having a large amount of information are requiredto be transmitted in a narrow radio band in order to effectively andefficiently use the frequency band. It is thus necessary to consider theuse of a high-efficiency modulation method, such as a multilevelquadrature amplitude modulation (QAM), under a mobile radio propagationenvironment. However, a high-efficiency modulation method represented byQAM has poor noise-resistant characteristics and interference-resistantcharacteristics, and is also vulnerable to distortions.

In a receiving apparatus positioned far from a transmission station or abase station, or in a receiving apparatus receiving signals while it ismoving, the received signals have a low signal-to-noise (S/N) ratio orhave a propagation distortion. This easily causes degradation ofreceiving characteristics and consequent, failure to transmitinformation with a satisfactory quality. This problem becomes seriousparticularly for terrestrial television broadcasting that covers a verywide area. Accordingly, an effective improvement in the transmissionmethod is required.

A typical measure being taken for improving the receivingcharacteristics under a severe multiple radio propagation environment orunder a poor mobile receiving environment is diversity reception. As adiversity reception method, antenna selection diversity reception isprimarily used in which digital signal sequences are demodulated byselecting from a plurality of receiving antennas the receiving antennahaving the maximum radio frequency (RF) signal power.

In the OFDM transmission method, however, a received signal isfrequency-converted into a baseband signal, which is further convertedinto frequency spectra in units of symbols. A digital signal sequence isthen demodulated from each line spectrum which forms a frequencyspectrum. Thus, the antenna selection diversity reception that changesantennas by RF signals is not very effective on the OFDM transmissionmethod. Particularly upon generation of severe frequency selectivefading, the receiving characteristics are completely different accordingto the line spectrum which forms a frequency spectrum. Thus, there is anincreasing demand for selection diversity that obtains optimal receivingcharacteristics for each line spectrum.

As discussed above, in a radio communication/broadcasting system fortransmitting digital signal sequences by using the OFDM method, it isnecessary to transmit high-quality and high-definition information inall the places within a wide area covered by this system whileeffectively utilizing the multipath-resistant transmissioncharacteristics of the OFDM transmission method. To satisfy the aboverequirements, it is further necessary to inhibit the degradation of thereceiving characteristics under a severe multiple reflection wavepropagation (multipath) environment and to suppress the degradation ofthe receiving characteristics while a receiving apparatus is moving.

In particular, in future multimedia communications and digitalterrestrial broadcasting, in which image transmission is to become themainstream, the application of a high-efficiency modulation method, suchas multilevel QAM, will be essential. Thus, there is an increasingdemand for an improvement in the receiving characteristics in the OFDMtransmission method, in particular, for the development of an OFDMdiversity receiving apparatus that exhibits good receivingcharacteristics even in response to frequency selective fading.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an OFDMdiversity receiving apparatus for use in an OFDM radiocommunication/broadcasting system that suppresses the degradation of thereceiving characteristics under a multiple reflection wave propagationenvironment or under a moving reception environment.

To achieve the above object, the present invention provides a diversityreceiving apparatus for use in a radio communication system that uses anOFDM technique. The diversity receiving apparatus includes a pluralityof diversity branches. Each of the diversity branches has a receiver forreceiving an OFDM signal via an antenna and outputting the OFDM receivedsignal, a first transformer for transforming the OFDM received signalinto a frequency spectrum and outputting the OFDM received signalfrequency spectrum, and a channel frequency response calculating unitfor calculating a channel frequency response in accordance with the OFDMreceived signal frequency spectrum and a reference frequency spectrum. Aselector unit selects the diversity branch that has generated the OFDMreceived signal frequency spectrum having a maximum amplitude or amaximum power.

The present invention also provides a diversity receiving apparatus foruse in a radio communication system that uses an OFDM technique. Thediversity receiving apparatus includes a plurality of diversitybranches. Each of the diversity branches has a receiver for receiving anOFDM signal via an antenna and outputting the OFDM received signal, afirst transformer for transforming the OFDM received signal into afrequency spectrum and outputting the OFDM received signal frequencyspectrum, and a channel frequency response calculating unit forcalculating a channel frequency response in accordance with the OFDMreceived signal frequency spectrum and a reference frequency spectrum. Aselector unit selects the diversity branch that has generated thechannel frequency response having a maximum amplitude or a maximumpower.

More specifically, a first aspect of the present invention provides adiversity receiving apparatus for use in a radio communication systemthat uses an OFDM technique. The diversity receiving apparatus includesa plurality of receivers, each of the receivers for receiving an OFDMsignal via an antenna and outputting the OFDM received signal. Each of aplurality of first transformers transforms the OFDM received signal intoa frequency spectrum. At least one reference frequency spectrumgenerator generates a reference frequency spectrum relative to the OFDMreceived signal. Each of a plurality of channel frequency responsecalculating units calculates a channel frequency response in accordancewith the frequency spectrum output from each of the first transformersand the reference frequency spectrum. A distortion compensatorcompensates for a distortion of the frequency spectrum output from eachof the first transformers by using the corresponding channel frequencyresponse. The frequency spectra output from the first transformers areinput into a selector unit as first input signals, while thedistortion-compensated frequency spectra output from the distortioncompensator are input into the selector unit as second input signals.The selector unit then selects and outputs the second input signalcorresponding to the first input signal having a maximum amplitude or amaximum power. A demodulator demodulates the output signal of theselector unit into a digital signal sequence.

A second aspect of the present invention provides a diversity receivingapparatus for use in a radio communication system that uses an OFDMtechnique. The diversity receiving apparatus includes a plurality ofreceivers, each of the receivers for receiving an OFDM signal via anantenna and outputting the OFDM received signal. Each of a plurality offirst transformers transforms the OFDM received signal into a frequencyspectrum. At least one reference frequency spectrum generator generatesa reference frequency spectrum relative to the OFDM received signal.Each of a plurality of channel frequency response calculating unitscalculates a channel frequency response in accordance with the frequencyspectrum output from each of the first transformers and the referencefrequency spectrum. A distortion compensator compensates for adistortion of the frequency spectrum output from each of the firsttransformers by using the corresponding channel frequency response. Thechannel frequency responses calculated by the channel frequency responsecalculating units are input into a selector unit as first input signals,while the distortion-compensated frequency spectra output from thedistortion compensator are input into the selector unit as second inputsignals. The selector unit then selects and outputs the second inputsignal corresponding to the first input signal having a maximumamplitude or a maximum power. A demodulator demodulates the outputsignal of the selector unit into a digital signal sequence.

A third aspect of the present invention provides a diversity receivingapparatus for use in a radio communication system that uses an OFDMtechnique. The diversity receiving apparatus includes a plurality ofreceivers, each of the receivers for receiving an OFDM signal via anantenna and outputting the OFDM received signal. Each of a plurality offirst transformers transforms the OFDM received signal into a frequencyspectrum. At least one reference frequency spectrum generator generatesa reference frequency spectrum relative to the OFDM received signal.Each of a plurality of channel frequency response calculating unitscalculates a channel frequency response in accordance with the frequencyspectrum output from each of the first transformers and the referencefrequency spectrum. A distortion compensator compensates for adistortion of the frequency spectrum output from each of the firsttransformers by using the corresponding channel frequency response. Aselector unit selects and outputs, among the distortion-compensatedfrequency spectra output from the distortion compensator, thedistortion-compensated frequency spectrum having a maximum amplitude ora maximum power. A demodulator demodulates the output signal of theselector unit into a digital signal sequence.

Upon generation of frequency selective fading, the receivingcharacteristics are completely different according to the line spectrumwhich forms a frequency spectrum. Even under this environment, accordingto the first, second, and third aspects of the present invention,diversity reception that obtains optimal receiving characteristics foreach line spectrum can be achieved. It is thus possible to effectivelysuppress the degradation of the receiving characteristics caused under amultiple reflection propagation environment or a mobile receivingenvironment.

A fourth aspect of the present invention provides a diversity receivingapparatus for use in a radio communication system that uses an OFDMtechnique. The diversity receiving apparatus includes a plurality ofreceivers, each of the receivers for receiving an OFDM signal via anantenna and outputting the OFDM received signal. Each of a plurality offirst transformers transforms the OFDM received signal into a frequencyspectrum. At least one reference frequency spectrum generator generatesa reference frequency spectrum relative to the OFDM received signal.Each of a plurality of channel frequency response calculating unitscalculates a channel frequency response in accordance with the frequencyspectrum output from each of the first transformers and the referencefrequency spectrum. The frequency spectra output from the firsttransformers are input into a selector unit as first input signals,while the channel frequency responses calculated by the channelfrequency response calculating units are input into the selector unit assecond input signals. The selector unit then selects the first inputsignal having a maximum amplitude or a maximum power and the secondinput signal corresponding to the selected first input signal. Theselector unit further outputs the selected first input signal and theselected second input signal as a first output signal and a secondoutput signal, respectively. A distortion compensator compensates for adistortion of the first output signal of the selector unit by using thesecond output signal of the selector unit. A demodulator demodulates theoutput signal of the distortion compensator into a digital signalsequence.

A fifth aspect of the present invention provides a diversity receivingapparatus for use in a radio communication system that uses an OFDMtechnique. The diversity receiving apparatus includes a plurality ofreceivers, each of the receivers for receiving an OFDM signal via anantenna and outputting the OFDM received signal. Each of a plurality offirst transformers transforms the OFDM received signal into a frequencyspectrum. At least one reference frequency spectrum generator generatesa reference frequency spectrum relative to the OFDM received signal.Each of a plurality of channel frequency response calculating unitscalculates a channel frequency response in accordance with the frequencyspectrum output from each of the first transformers and the referencefrequency spectrum. The channel frequency responses calculated by thechannel frequency response calculating units are input into a selectorunit as first input signals, while the frequency spectra output from thefirst transformers are input into the selector unit as second inputsignals. The selector unit then selects the first input signal having amaximum amplitude or a maximum power and the second input signalcorresponding to the selected first input signal. The selector unitfurther outputs the selected first input signal and the selected secondinput signal as a first output signal and a second output signal,respectively. A distortion compensator compensates for a distortion ofthe second output signal of the selector unit by using the first outputsignal of the selector unit. A demodulator demodulates the output signalof the distortion compensator into a digital signal sequence.

Upon generation of frequency selective fading, the receivingcharacteristics are completely different according to the line spectrumwhich forms a frequency spectrum. Even under this environment, accordingto the first, second, and third aspects of the present invention,diversity reception that obtains optimal receiving characteristics foreach line spectrum can be achieved. It is thus possible to effectivelyinhibit the degradation of the receiving characteristics caused under amultiple reflection propagation environment or a mobile receivingenvironment. Additionally, only a single distortion compensator isrequired.

According to the first, second, or third aspect of the presentinvention, a filtering unit for filtering the channel frequency responsecalculated by each of the channel frequency response calculating unitsmay be disposed between each of the channel frequency responsecalculating units and the distortion compensator. Similarly, accordingto the fourth or fifth aspect of the present invention, a filtering unitfor filtering the channel frequency response calculated by each of thechannel frequency response calculating units may be disposed betweeneach of the channel frequency response calculating units and theselector unit. By the provision of the filtering unit, noise componentscontained in the channel frequency responses can be eliminated, therebyfurther enhancing the receiving characteristics.

The above filtering unit may include a filtering-bandwidth-variablefilter that receives the channel frequency response calculated by eachof the channel frequency response calculating units, a secondtransformer for transforming the channel frequency response into achannel impulse response, a propagation delay period measuring portionfor measuring, based on the channel impulse response, the propagationdelay period caused under a multiple reflection propagation environment,and a filtering bandwidth setting portion for setting the filteringbandwidth of the filter based on the measurements by the propagationdelay period measuring portion. With this arrangement, the propagationdelay period of a multiple reflection propagation environment can bemeasured, thereby setting the bandwidth of the filter in response to thepropagation delay period. As a result, noise components included in thechannel frequency response of the propagation delay period can beefficiently eliminated.

The present invention may further include a re-modulator forre-modulating the digital signal sequence demodulated by the demodulatorand for generating a re-modulated frequency spectrum, and a frequencyspectrum selector for selectively outputting the re-modulated frequencyspectrum or the reference frequency spectrum to the channel frequencyresponse calculating units.

In a communication system or a broadcasting system in which digitalsignal sequences are transmitted according to an OFDM transmissionmethod using a slot, the frequency spectrum selector may select thereference frequency spectrum when receiving an OFDM signal of a knowndata sequence contained in the head of the slot. When receiving the OFDMsignals of the other data sequences, the frequency spectrum selector mayselect the re-modulated frequency spectrum. Upon calculating the channelfrequency response by using the re-modulated frequency spectrum, thedistortions of the frequency spectrum can be compensated by using themost updated channel frequency response. It is thus possible to improvethe receiving characteristics even under a dynamic propagationenvironment.

According to the present invention, the selector unit may include aplurality of synthesizers for synthesizing the amplitudes or the powerof line spectra forming the first input signal, and a comparator forcomparing outputs of the synthesizers, thereby performing a selectingoperation based on a comparison result of the comparator. With thisarrangement, the diversity branch that exhibits excellent receivingcharacteristics can be easily selected, thereby effectively enhancingthe receiving characteristics.

Alternatively, the selector unit may include a comparator for comparing,among line spectra forming the first input signal, the amplitude or thepower of the line spectra of the same frequency, thereby performing aselecting operation based on a comparison result of the comparator. Thismakes it possible to easily select the diversity branch that exhibitsexcellent receiving characteristics for each line spectrum, therebyimproving the receiving characteristics even more effectively.

A sixth aspect of the present invention provides a method for receivinga diversity signal for use in a radio communication system that uses anOFDM technique. The method includes the steps of: providing a pluralityof diversity branches, each of the diversity branches including areceiver for receiving an OFDM signal via an antenna and outputting theOFDM received signal, a first transformer for transforming the OFDMreceived signal into a frequency spectrum and outputting the OFDMreceived signal frequency spectrum, and a channel frequency responsecalculating unit for calculating a channel frequency response inaccordance with the OFDM received signal frequency spectrum and areference frequency spectrum; and selecting the diversity branch thathas generated the OFDM received signal frequency spectrum having amaximum amplitude or a maximum power.

A seventh aspect of the present invention provides a method forreceiving a diversity signal for use in a radio communication systemthat uses an OFDM technique. The method includes the steps of: providinga plurality of diversity branches, each of the diversity branchesincluding a receiver for receiving an OFDM signal via an antenna andoutputting the OFDM received signal, a first transformer fortransforming the OFDM received signal into a frequency spectrum andoutputting the OFDM received signal frequency spectrum, and a channelfrequency response calculating unit for calculating a channel frequencyresponse in accordance with the OFDM received signal frequency spectrumand a reference frequency spectrum; and selecting the diversity branchthat has generated the channel frequency response having a maximumamplitude or a maximum power.

An eighth aspect of the present invention provides a method forreceiving a diversity signal for use in a radio communication systemthat uses an OFDM technique. The method includes the steps of receivingan OFDM signal via an antenna and outputting the OFDM received signal;transforming the OFDM received signal into a frequency spectrum;generating a reference frequency spectrum relative to the OFDM receivedsignal; calculating a channel frequency response in accordance with thefrequency spectrum and the reference frequency spectrum; compensatingfor a distortion of the frequency spectrum by using the correspondingchannel frequency response; selecting and outputting, if the frequencyspectra are determined as first input signals and thedistortion-compensated frequency spectra are determined as second inputsignals, the second input signal corresponding to the first input signalhaving a maximum amplitude or a maximum power; and demodulating theoutput signal of the selecting step into a digital signal sequence.

A ninth aspect of the present invention, there is provided a method forreceiving a diversity signal for use in a radio communication systemthat uses an OFDM technique. The method includes the steps of receivingan OFDM signal via an antenna and outputting the OFDM received signal;transforming the OFDM received signal into a frequency spectrum;generating a reference frequency spectrum relative to the OFDM receivedsignal; calculating a channel frequency response in accordance with thefrequency spectrum and the reference frequency spectrum; compensatingfor a distortion of the frequency spectrum by using the correspondingchannel frequency response; selecting and outputting, if the channelfrequency responses are determined as first input signals and thedistortion-compensated frequency spectra are determined as second inputsignals, the second input signal corresponding to the first input signalhaving a maximum amplitude or a maximum power; and demodulating theoutput signal of the selecting step into a digital signal sequence.

A tenth aspect of the present invention provides a method for receivinga diversity signal for use in a radio communication system that uses anOFDM technique. The method includes the steps of receiving an OFDMsignal via an antenna and outputting the OFDM received signal;transforming the OFDM received signal into a frequency spectrum;generating a reference frequency spectrum relative to the OFDM receivedsignal; calculating a channel frequency response in accordance with thefrequency spectrum and the reference frequency spectrum; compensatingfor a distortion of the frequency spectrum by using the correspondingchannel frequency response; selecting and outputting, among thedistortion-compensated frequency spectra, the distortion-compensatedfrequency spectrum having a maximum amplitude or a maximum power; anddemodulating the output signal of the selecting step into a digitalsignal sequence.

An eleventh aspect of the present invention provides a method forreceiving a diversity signal for use in a radio communication systemthat uses an OFDM technique. The method includes the steps of receivingan OFDM signal via an antenna and outputting the OFDM received signal;transforming the OFDM received signal into a frequency spectrum;generating a reference frequency spectrum relative to the OFDM receivedsignal; calculating a channel frequency response in accordance with thefrequency spectrum and the reference frequency spectrum; selecting, ifthe frequency spectra are determined as first input signals and thechannel frequency responses are determined as second input signals, thefirst input signal having a maximum amplitude or a maximum power and thesecond input signal corresponding to the selected first input signal,and outputting the selected first input signal and the selected secondinput signal as a first output signal and a second output signal,respectively; compensating for a distortion of the first output signalby using the second output signal; and demodulating the output signal ofthe compensating step into a digital signal sequence.

A twelfth aspect of the present invention provides a method forreceiving a diversity signal for use in a radio communication systemthat uses an OFDM technique. The method includes the steps of receivingan OFDM signal via an antenna and outputting the OFDM received signal;transforming the OFDM received signal into a frequency spectrum;generating a reference frequency spectrum relative to the OFDM receivedsignal; calculating a channel frequency response in accordance with thefrequency spectrum and the reference frequency spectrum; selecting, ifthe channel frequency responses are determined as first input signalsand the frequency spectra are determined as second input signals, thefirst input signal having a maximum amplitude or a maximum power and thesecond input signal corresponding to the selected first input signal,and outputting the selected first input signal and the selected secondinput signal as a first output signal and a second output signal,respectively; compensating for a distortion of the second output signalby using the first output signal; and demodulating the output signal ofthe compensating step into a digital signal sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an OFDM diversity receivingapparatus according to a first embodiment of the present invention;

FIG. 2 is a block diagram illustrating an example of the configurationof a selector unit for use in an OFDM diversity receiving apparatusaccording to the present invention;

FIG. 3 is a block diagram illustrating another example of theconfiguration of a selector unit for use in an OFDM diversity receivingapparatus according to the present invention;

FIG. 4 is a block diagram illustrating an OFDM diversity receivingapparatus according to a second embodiment of the present invention;

FIG. 5 is a block diagram illustrating an OFDM diversity receivingapparatus according to a third embodiment of the present invention;

FIG. 6 is a block diagram illustrating an OFDM diversity receivingapparatus according to a fourth embodiment of the present invention;

FIG. 7 is a block diagram illustrating an OFDM diversity receivingapparatus according to a fifth embodiment of the present invention;

FIG. 8 is a block diagram illustrating an OFDM diversity receivingapparatus according to a sixth embodiment of the present invention;

FIG. 9 is a block diagram illustrating an OFDM diversity receivingapparatus according to a seventh embodiment of the present invention;

FIG. 10 is a block diagram illustrating an OFDM diversity receivingapparatus according to an eighth embodiment of the present invention;

FIG. 11 is a block diagram illustrating a filter unit for use in an OFDMdiversity receiving apparatus according to the present invention;

FIGS. 12A and 12B illustrate the operation of the filtering unit shownin FIG. 11;

FIG. 13 is a block diagram illustrating an OFDM diversity receivingapparatus according to a ninth embodiment of the present invention; and

FIG. 14 is a diagram illustrating receiving characteristics of an OFDMdiversity receiving apparatus according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention are described indetail below with reference to the drawings.

First Embodiment

Referring to the block diagram illustrating an OFDM diversity receivingapparatus according to a first embodiment of the present invention inFIG. 1, a number n of diversity branches (“branches”) #1 through #n aredisposed. In the respective branches #1 through #n, OFDM signals arereceived by receiving antennas 1 through 3 and are input into receivers4 through 6, respectively. Each of the receivers 4 through 6 has basicfunctions, such as amplification, frequency mixing, and filtering, thatare required for frequency-converting an RF-band OFDM signal into abaseband signal, and also has functions, such as synchronization,frequency correction, and guard interval removal, that are inherent tothe OFDM transmission method. The baseband signal from which the guardinterval has been removed is then output as an OFDM received signal.

The OFDM received signals output from the receivers 4 through 6 areinput into first transformers 7 through 9, respectively, and aretransformed into frequency spectra (“OFDM received signal frequencyspectra”) by transformation processing represented by fast Fouriertransform (FFT).

Reference frequency spectrum generators 10 through 12 respectivelygenerate reference frequency spectra relative to the OFDM receivedsignal frequency spectra output from the first transformers 7 through 9.Channel frequency response calculating units 13 through 15 respectivelycalculate channel frequency responses inherent in the branches #1through #n by using the OFDM received signal frequency spectra and thereference frequency spectra.

The OFDM received signal frequency spectra generated in the firsttransformers 7 through 9 are respectively input into distortioncompensators 16 through 18, and the distortions included in the OFDMreceived signal frequency spectra caused by the multiple reflectionpropagation environment are compensated for by using the channelfrequency responses of the branches #1 through #n calculated in thechannel frequency response calculating units 13 through 15,respectively. The OFDM received signal frequency spectra generated inthe first transformers 7 through 9 are input into a selector unit 19 asfirst input signals A, while the distortion-compensated frequencyspectra generated in the distortion compensators 16 through 18 are inputinto the selector unit 19 as second input signals B.

The selector unit 19 then compares the OFDM received signal frequencyspectra generated in the first transformers 7 through 9 of the branches#1 through #n, i.e., compares the first input signals A. Uponcomparison, the selector unit 19 first selects the branch that hasgenerated the frequency spectrum having the maximum amplitude or themaximum power, and then selects the distortion-compensated OFDM receivedsignal frequency spectrum output from the selected branch and input intothe selector unit 19. The selected distortion-compensated OFDM receivedsignal frequency spectrum is input into a demodulator 20 and isdemodulated into a digital signal sequence. The selector unit 19 isdescribed in detail below.

In this embodiment, the selector unit 19 may select the branch bycomparing the OFDM received signal frequency spectra included in eachsymbol of the OFDM received signal output from each of the firsttransformers 7 through 9. If, however, the propagation environment doesnot change sharply with a lapse of time, the branch may be selected byusing the OFDM received signal frequency spectra that were calculatedwithin a predetermined time before the selection. In this case, it isalmost possible to fix a branch to be selected in advance, or tosuitably set a branch to be selected in a few units of symbols.

According to the first embodiment, the branch that has generated theOFDM received signal frequency spectrum having the maximum power or themaximum amplitude is selected from a plurality of branches #1 through#n. Then, the distortion-compensated OFDM received signal frequencyspectrum obtained in the selected branch is demodulated into a digitalsignal sequence. As a consequence, receiving characteristics can beenhanced.

(Selector unit 19)

An example of the configuration of the selector unit 19 is discussedbelow with reference to FIG. 2.

The first input signals A output from the branches #1 through #n arerespectively input into synthesizers 41 through 43 of the selector unit19. In the first embodiment, the first input signals A are equivalent tothe OFDM received signal frequency spectra output from the firsttransformers 7 through 9. In some of the subsequent embodimentsdescribed below, however, the channel frequency responses output fromthe channel frequency response calculating units 13 through 15 are usedas the first input signals A. Generally, the first transformers 7through 9 each perform FFT to transform the OFDM received signal intothe OFDM received signal frequency spectrum. Thus, the frequencyspectrum is formed of a plurality of discrete line spectra, and thechannel frequency response calculated by using the OFDM received signalfrequency spectrum is also formed of a plurality of discrete linespectra.

Each of the synthesizers 41 through 43 synthesizes the amplitude or thepower of a plurality of line spectra of the first input signal A andoutputs the synthesized amplitude or power. In this case, each of thesynthesizers 41 through 43 generally synthesizes the amplitude or thepower of line spectra included in one symbol of an OFDM received signal.However, the synthesizers 41 through 43 may synthesize a few linespectra or line spectra included in a few symbols of an OFDM receivedsignal. The outputs of the synthesizers 41 through 43 are input into acomparator 44 and a selector 45.

The comparator 44 compares the outputs of the synthesizers 41 through 43corresponding to the branches #1 through #n and identifies the branchthat has generated the maximum output. The comparator 44 then informsthe selector 45 of the identified branch number. The selector 45transmits the output signal, which serves as a first output signal, ofthe synthesizer corresponding to the branch number informed by thecomparator 44.

As discussed above, the selector unit 19 identifies the branch that hasgenerated the output having the maximum amplitude or the maximum powersynthesized from a plurality of line spectra of the first input signalA. The selector unit 19 then selects the first input signal A outputfrom the identified branch, and outputs it to the demodulator 20. It isthus possible to identify the optimal branch from the average receivingcharacteristics of a plurality of line spectra in each of the branches#1 through #n, and to select the first input signal A from theidentified branch. As a result, the receiving characteristics can beenhanced more effectively.

Another example of the configuration of the selector unit 19 isdescribed below with reference to FIG. 3.

FIG. 3 illustrates the operation of the selector unit 19 when twobranches are used. The first input signal A is formed of m number ofdiscrete line spectra of frequencies f₁ through f_(m). The selector unit19 compares the amplitude or the power of the line spectra of the samefrequency of the first input signals A. Then, the selector unit 19selects the branch that has output the line spectrum having the maximumamplitude or the maximum power of each of the frequencies f₁ throughf_(m).

According to the above configuration of the selector unit 19, the branchthat has output the line spectrum having the maximum amplitude or themaximum power of each frequency can be selected, thereby significantlyimproving the receiving characteristics.

Second Embodiment

FIG. 4 is a block diagram illustrating an OFDM diversity receivingapparatus according to a second embodiment of the present invention.

The second embodiment is constructed substantially similarly to thefirst embodiment, except that a single reference-frequency-spectrumgenerator 21 is shared among the respective branches #1 through #n,because all the reference frequency spectra generated in the referencefrequency spectrum generators 10 through 12 are the same. Accordingly,an improvement in the receiving characteristics similar to that offeredby the first embodiment can be achieved.

In the second embodiment, as well as in the first embodiment, theselector unit 19 may select the branch by comparing the OFDM receivedsignal frequency spectra included in each symbol of an OFDM receivedsignal output from each of the first transformers 7 through 9.Alternatively, if the propagation environment does not changedrastically during a period of time, the branch may be selected by usingthe OFDM received signal frequency spectra that were calculated within apredetermined time before the selection.

Third Embodiment

FIG. 5 is a block diagram illustrating an OFDM diversity receivingapparatus according to a third embodiment of the present invention. Inthe third embodiment, as well as in the first embodiment, the branches#1 through #n generate channel frequency responses anddistortion-compensated OFDM received signal frequency spectra. Thechannel frequency responses produced in the channel frequency responsecalculating units 13 through 15 are directly input into the selectorunit 26 as first input signals A, while the distortion-compensated OFDMreceived signal frequency spectra generated in the distortioncompensators 16 through 18 are input into the selector unit 26 as secondinput signals B.

The selector unit 26 compares the channel frequency responses generatedin the branches #1 through #n, i.e., compares the first input signals A,and selects the branch that has generated the channel frequency responsehaving the maximum amplitude or the maximum power. The selector unit 26then selects the distortion-compensated OFDM received signal frequencyspectrum, i.e., the second input signal B, from the selected branch, andthe selected frequency spectrum is further demodulated into a digitalsignal sequence in the demodulator 20.

According to the third embodiment, it is possible to select the branchthat has generated the channel frequency response having the maximumamplitude or the maximum power from a plurality of branches #1 through#n. Then, a digital signal sequence is demodulated from thedistortion-compensated frequency spectrum obtained in the selectedbranch. As a result, receiving characteristics are considerablyimproved.

In this embodiment, as well as in the second embodiment illustrated inFIG. 4, a single reference-frequency-spectrum generator 21 is sharedamong the branches #1 through #n. However, individualreference-frequency-spectrum generators may be provided for therespective branches #1 through #n, as in the first embodiment shown inFIG. 1.

In the third embodiment, as well as in the first embodiment, if thepropagation environment does not change drastically during a period oftime, the branch may be selected by using the channel frequencyresponses calculated within a predetermined time before the selection.Accordingly, instead of selecting the branch by using the channelfrequency responses in each symbol of the OFDM received signal, it isalmost possible to fix a branch to be selected in advance, or tosuitably set a branch to be selected in a few units of symbols.

Fourth Embodiment

FIG. 6 is a block diagram illustrating an OFDM diversity receivingapparatus according to a fourth embodiment of the present invention. Inthis embodiment, instead of the OFDM received signal frequency spectraoutput from the first transformers 7 through 9 in the second embodiment,distortion-compensated OFDM received signal frequency spectra outputfrom the distortion compensators 16 through 18 are input into theselector unit 27 as first input signals A. Accordingly, only the firstinput signals A are input into the selector unit 27 from the branches #1through #n.

More specifically, in the branches #1 through #n, the OFDM signalsreceived by the receiving antennas 1 through 3 are respectivelyconverted into baseband signals from which guard intervals are removedin the receivers 4 through 6. The baseband signals are then input intothe first transformers 7 through 9 and are transformed into OFDMreceived signal frequency spectra by. performing transformationprocessing, such as FFT.

The OFDM received signal frequency spectra output from the firsttransformers 7 through 9 are input into the channel frequency responsecalculating units 13 through 15 and further into the distortioncompensators 16 through 18. The channel frequency response calculatingunits 13 through 15 calculate channel frequency responses by using theOFDM received signal frequency spectra and the reference frequencyspectra generated in the reference frequency spectrum generator 21.Subsequently, the distortion compensators 16 through 18 compensate forthe distortions of the OFDM received signal frequency spectra by usingthe channel frequency responses.

Generally, for an amplitude modulation signal, such as a QAM signal, thedistortion compensators 16 through 18 divide the OFDM received signalfrequency spectra output from the first transformers 7 through 9 by thechannel frequency responses, respectively, thereby compensating forfrequency selective fading caused under a multiple response propagationenvironment. On the other hand, for a phase modulation signal, such as aquadrature phase shift keying (QPSK) signal, phase distortion iscompensated for by using a multiplier rather than a divider, whichinevitably increases the scale of the apparatus. More specifically, thedistortion compensators 16 through 18 respectively compensate for phasedistortions by multiplying the OFDM received signal frequency spectraoutput from the first transformers 7 through 9 with conjugate complexsignals of the channel frequency responses. In this case, the amplitudesof the distortion-compensated OFDM received signal frequency spectraoutput from the distortion compensators 16 through 18 are proportionalto the amplitude components of the OFDM received signal frequencyspectra output from the first transformers 7 through 9. Thus, theselector unit 27 may select the branch by comparing the amplitudes orthe power of the distortion-compensated OFDM received signal frequencyspectra rather than comparing the OFDM received signal frequency spectraoutput from the first transformers 7 through 9.

Accordingly, in this embodiment, the selector unit 27 compares thedistortion-compensated OFDM received signal frequency spectra input asthe first input signals A and selects the branch that has generated thedistortion-compensated OFDM received signal frequency spectrum havingthe maximum amplitude or the maximum power. Thus, thedistortion-compensated OFDM received signal frequency spectrum input asthe first input signal A from the selected branch, i.e., thedistortion-compensated OFDM received signal frequency spectrum havingthe maximum amplitude or the maximum power, is output from the selectorunit 27. The frequency spectrum is then demodulated into a digitalsignal sequence by the demodulator 20.

In this embodiment, the selector unit 27 compares thedistortion-compensated OFDM received signal frequency spectra in eachsymbol of the OFDM received signal input from each branch. If, however,the propagation environment does not change drastically during a periodof time, the branch may be selected by using the distortion-compensatedOFDM received signal frequency spectra that were calculated within apredetermined time before the selection. Additionally, individualreference-frequency-spectrum generators may be provided for therespective branches #1 through #n, as in the first embodimentillustrated in FIG. 1.

Fifth Embodiment

FIG. 7 is a block diagram illustrating an OFDM diversity receivingapparatus according to a fifth embodiment of the present invention. Inthis embodiment, after the selector unit 19 has selected the branch, thedistortion of the OFDM received signal frequency spectrum output fromthe selected branch is compensated for by using the channel frequencyresponse of the selected branch.

More specifically, in the branches #1 through #n, the OFDM signalsreceived by the receiving antennas 1 through 3 are respectivelyconverted into baseband signals in the receivers 4 through 6. Thebaseband signals are further transformed into OFDM received signalfrequency spectra by the first transformers 7 through 9. The channelfrequency response calculating units 13 through 15 calculate channelfrequency responses from the OFDM received signal frequency spectra andthe reference frequency spectrum generated by the reference frequencyspectrum generator 21. The OFDM received signal frequency spectra areinput into the selector unit 19 as first input signals A, while thechannel frequency responses are input into the selector unit 19 assecond input signals B.

The selector unit 19 then compares the OFDM received signal frequencyspectra, which serve as the first input signals A, generated in thebranches #1 through #n and selects the branch that has generated theOFDM received signal frequency spectrum having the maximum amplitude orthe maximum power. Then, the OFDM received signal frequency spectrum andthe channel frequency response which are output from the selected branchare input into a distortion compensator 22. The selector unit 19 may beconfigured similarly to that shown in FIG. 2 or 3.

The distortion compensator 22 compensates for the distortion of the OFDMreceived signal frequency spectrum output from the selector unit 19 byusing the channel frequency response output from the selector unit 19.The distortion-compensated OFDM received signal frequency spectrumoutput from the distortion compensator 22 is demodulated into a digitalsignal sequence in the demodulator 20.

In this manner, according to this embodiment, the branch that hasgenerated the OFDM received signal frequency spectrum having the maximumamplitude or the maximum power is selected from a plurality of branches#1 through #n. The distortion of the OFDM received signal frequencyspectrum is then compensated for by using the channel frequency responseobtained in the selected branch, and the distortion-compensatedfrequency spectrum is demodulated into a digital signal sequence. Withthis arrangement, receiving characteristics can be improved.Additionally, only a single distortion compensator is required in thisembodiment, thereby miniaturizing the receiving apparatus compared tothose of the foregoing embodiments.

In this embodiment, a single reference-frequency-spectrum generator 21is shared among the branches #1 through #n. However, individualreference-frequency-spectrum generators may be provided for therespective branches #1 through #n, as in the first embodimentillustrated in FIG. 1.

In the fifth embodiment, as well as in the first through fourthembodiments, the selector unit 19 may select the branch by comparing theOFDM received signal frequency spectra or the channel frequencyresponses in each symbol of an OFDM received signal. Alternatively, thebranch may be selected by using the OFDM received signal frequencyspectra or the channel frequency responses that were calculated within apredetermined time before the selection.

Sixth Embodiment

FIG. 8 is a block diagram illustrating an OFDM diversity receivingapparatus according to a sixth embodiment of the present invention. Inthis embodiment, as well as in the fifth embodiment shown in FIG. 7,only a single distortion compensator is required due to the followingconfiguration. After the selector unit 19 has selected the branch, thedistortion of the OFDM received signal frequency spectrum output fromthe selected branch is compensated for by using the channel frequencyresponse of the selected branch.

More specifically, as in the fifth embodiment, in the branches #1through #n, the OFDM received signal frequency spectra are generated inthe first transformers 7 through 9, and the channel frequency responsesare generated in the channel frequency response calculating units 13through 15. Unlike the fifth embodiment, however, the channel frequencyresponses are input into the selector unit 19 as first input signals A,while the OFDM received signal frequency spectra are input into theselector unit 19 as second input signals B.

The selector unit 19 compares the channel frequency responses generatedin the branches #1 through #n as the first input signals A and selectsthe branch that has generated the channel frequency response having themaximum amplitude or the maximum power. The selector unit 19 may beconfigured substantially similarly to that shown in FIG. 2 or 3. TheOFDM received signal frequency spectrum and the channel frequencyresponse which are output from the selected branch are input into thedistortion compensator 22.

The distortion compensator 22 compensates for the distortion of the OFDMreceived signal frequency spectrum output from the selector unit 19 byusing the channel frequency response output from the selector unit 19.The distortion-compensated OFDM received signal frequency spectrumoutput from the distortion compensator 22 is then demodulated into adigital signal sequence in the demodulator 20.

According to this embodiment, the branch that has generated the channelfrequency response having the maximum power or the maximum amplitude isselected from a plurality of branches #1 through #n. The distortion ofthe OFDM received signal frequency spectrum of the selected branch isthen compensated for by using the channel frequency response obtained inthe selected branch, and the distortion-compensated OFDM received signalfrequency spectrum is demodulated into a digital signal sequence. As aresult, the receiving characteristics can be enhanced. Additionally, thesixth embodiment, as well the fifth embodiment, requires only a singledistortion compensator, thereby reducing the size of the receivingapparatus.

Seventh Embodiment

FIG. 9 is a block diagram illustrating an OFDM diversity receivingapparatus according to a seventh embodiment of the present invention. Inthis embodiment, filtering units 23 through 25 are added to the secondembodiment shown in FIG. 4 for removing distortions, such as noise, fromchannel responses. More specifically, the filtering units 23 through 25are respectively interposed between the channel frequency responsecalculating units 13 through 15 and the distortion compensators 16through 18.

In the branches #1 through #n of this embodiment, the OFDM receivedsignal frequency spectra are calculated in the first transformers 7through 9 in a manner similar to the first embodiment. The channelfrequency response calculating units 13 through 15 then calculate thechannel frequency responses inherent in the branches #1 through #n byusing the OFDM received signal frequency spectra and the referencefrequency spectrum generated in the reference frequency spectrumgenerator 21. Subsequently, the channel frequency responses arerespectively input into the filtering units 23 through 25 in whichdistortions, such as noise, generated in the receivers 4 through 6 areremoved from the channel frequency responses. The distortion-freechannel frequency responses are then input into the distortioncompensators 16 through 18, which respectively compensates for thedistortions included in the OFDM received signal frequency spectraoutput from the first transformers 7 through 9 by using the filteredchannel frequency responses.

The OFDM received signal frequency spectra generated in the firsttransformers 7 through 9 are input into the selector unit 19 as firstinput signals A, while the distortion-compensated OFDM received signalfrequency spectra output from the distortion compensators 16 through 18are input into the selector unit 19 as second input signals B. Theselector unit 19 selects the branch in a manner similar to thecounterpart of the first embodiment shown in FIG. 1, and outputs thedistortion-compensated OFDM received signal frequency spectrum of theselected branch. The distortion-compensated OFDM received signalfrequency spectrum is then input into the demodulator 20 and isdemodulated into a digital signal sequence.

According to this embodiment, the filtering units 23 through 25 arerespectively interposed between the channel frequency responsecalculating units 13 through 15 and the distortion compensators 16through 18. With this arrangement, the channel frequency. responsescalculated in the branches #1 through #n can be formed more precise,thereby improving the receiving characteristics more effectively andaccurately.

Although in this embodiment the filtering units 23 through 25 are addedto the second embodiment illustrated in FIG. 4, filtering units may beadded to the third embodiment or the fourth embodiment shown in FIG. 5or 6. Additionally, in this embodiment, a singlereference-frequency-spectrum generator 21 is shared among the branches#1 through #n. However, individual reference-frequency-spectrumgenerators may be provided for the respective branches #1 through #n, asin the first embodiment shown in FIG. 1.

Eighth Embodiment

FIG. 10 is a block diagram illustrating an OFDM diversity receivingapparatus according to an eighth embodiment of the present invention. Inthis embodiment, as well as in the seventh embodiment shown in FIG. 9,filtering units 23 through 25 are added to the fifth embodiment shown inFIG. 7 for eliminating distortions, such as noise, from the channelfrequency responses. More specifically, the filtering units 23 through25 are respectively inserted between the channel frequency responsecalculating units 13 through and the selector unit 19.

In this embodiment, in the branches #1 through #n, OFDM received signalfrequency spectra are calculated in the first transformers 7 through 9in a manner similar to the first embodiment shown in FIG. 1. The channelfrequency response calculating units 13 through 15 respectivelycalculate the channel frequency responses inherent in the branches #1through #n by utilizing the OFDM received signal frequency spectra andthe reference frequency spectrum generated in the reference frequencyspectrum generator 21.

The channel frequency responses are then respectively input into thefiltering units 23 through 25, in which distortions, such as noise,produced in the receivers 4 through 6 are removed. The OFDM receivedsignal frequency spectra output from the first transformers 7 through 9are input into the selector unit 19 as first input signals A, while thedistortion-free channel frequency responses output from the filteringunits 23 through 25 are input into the selector unit 19 as second inputsignals B. The selector unit 19 selects the branch in a manner similarto the counterpart of the fifth embodiment shown in FIG. 7, and outputsthe OFDM received signal frequency spectrum and the distortion-freechannel frequency response which are input from the selected branch.

The distortion compensator 22 compensates for the distortions of theOFDM received signal frequency spectrum output from the selector unit 19by utilizing the channel frequency response output from the selectorunit 19. The demodulator 20 then demodulates the distortion-compensatedOFDM received signal frequency spectrum into a digital signal sequence.

According to the eighth embodiment, the filtering units 23 through 25are inserted between the channel frequency response calculating units 13through 15 and the selector unit 19. It is thus possible to form moreprecise channel frequency responses calculated in the branches #1through #n, thereby improving the receiving characteristics moreeffectively.

Although in this embodiment the filtering units 23 through 25 are addedto the fifth embodiment illustrated in FIG. 7, filtering units may beadded to the sixth embodiment shown in FIG. 8. Additionally, although asingle reference-frequency-spectrum generator 21 is shared among thebranches #1 through #n, individual reference-frequency-spectrumgenerators may be provided for the respective branches #1 through #n, asin the first embodiment shown in FIG. 1.

(Filtering Units)

FIG. 11 illustrates an example of the configuration of the filteringunits 23 through 25 used in the seventh embodiment or the eighthembodiment. Referring to FIG. 11, a filtering unit 50 represents one ofthe filtering units 23 through 25, which respectively serve as thesubsequent stages of the channel frequency response calculating units 13through 15 of the branches #1 through #n shown in FIG. 9 or 10. Thefiltering unit 50 is formed of a second transformer 51, a propagationdelay period measuring portion 52, a filtering bandwidth setting portion53, and a filter 54. The bandwidth of the filter 54 is variable.

The channel frequency response calculated in each of the branches #1through #n is input into the second transformer 51 and the filter 54.The second transformer 51 transforms the input channel frequencyresponse into time domain information, i.e., a channel impulse response,by performing transformation processing, such as inverse fast Fouriertransform (IFFT). The channel impulse response is generally referred toas “delay profile”, and represents the number of paths under a multipleradio propagation environment. The delay profile output from the secondtransformer 51 is then input into the propagation delay period measuringportion 52, in which the maximum delay period is measured from the delayprofile. Based on the measured maximum delay period, the bandwidth ofthe filter 54 is determined in the filtering bandwidth setting portion53. The channel frequency response is then filtered by the filter 54.

According to the filtering unit 50 configured as described above,distortions, such as noise, contained in the channel frequency responsescan be efficiently eliminated even under a propagation environment inwhich the delay period varies. This makes it possible to determine thechannel frequency responses more accurately, thereby improving thereceiving characteristics.

The operation of the second transformer 51 of the filtering unit 50shown in FIG. 11 is discussed in detail below with reference to FIGS.12A and 12B. FIG. 12A illustrates an example of a channel frequencyresponse calculated in each of the channel frequency responsecalculating units 13 through 15 of each branch under a 2-ray environmentmodel having a desired path signal and an undesired path signal. Under amultipath propagation environment, frequency selective fading occurs,and the distortions caused by the frequency selective fading vary with aspecific period in a frequency domain. Thus, the second transformer 51transforms the channel frequency response into time domain information,resulting in a delay profile, such as the one shown in FIG. 12B.Accordingly, the radio propagation environment can be assessed. It isthus possible to set a filtering bandwidth such that a desired pathsignal and an undesired path signal are included in a signal pass bandand the other components are blocked.

In this manner, the filtering bandwidth setting portion 53 first setsthe filtering bandwidth of the filter 54. With the determined bandwidth,the filter 54 filters the channel frequency response. The filtering unit50 thus passes only the required components and blocks the unnecessarycomponents, thereby making it possible to calculate channel frequencyresponses more accurately.

Ninth Embodiment

FIG. 13 is a block diagram illustrating an OFDM diversity receivingapparatus according to a ninth embodiment of the present invention. Theninth embodiment is configured in a manner similar to the eighthembodiment shown in FIG. 10, except that the following elements areadded. A re-modulator 31 re-modulates a digital signal sequence outputfrom the demodulator 20 so as to generate a re-modulated frequencyspectrum. A frequency spectrum selector 32 selects from the re-modulatedfrequency spectrum output from the re-modulator 31 and the referencefrequency spectrum output from the reference frequency spectrumgenerator 21. Delay units 33 through 35 are also added.

More specifically, according to the OFDM diversity receiving apparatusof this embodiment, in the branches #1 through #n, the OFDM signalsreceived by the receiving antennas 1 through 3 are respectively inputinto the receivers 4 through 6, and are converted into baseband signalsfrom which guard intervals are removed. The baseband signals are furtherinput into the first transformers 7 through 9 and are transformed intoOFDM received signal frequency spectra by performing transformationprocessing, such as FFT.

In a communication system or a broadcasting system in which digitalsignal sequences are transmitted according to an OFDM transmissionmethod using a slot, it is now assumed that a known data sequence isincluded at the head of the slot. With this configuration, when an OFDMsignal having such a known data sequence is received, a referencefrequency spectrum corresponding to the known data sequence is producedin the reference frequency spectrum generator 21 and is input intochannel frequency response calculating units 13 through 15 of therespective branches #1 through #n via the frequency spectrum selector32.

The outputs of the first transformers 7 through 9 of the branches #1through #n are respectively input into the delay units 33 through 35, inwhich the OFDM received signal frequency spectra are delayed for a unittime (symbol time). The provision of the delay units 33 through 35 isfor the purpose of synchronizing the OFDM received signal frequencyspectra with a unit time delay generated by re-modulating the digitalsignal sequence in the re-modulator 31. If the channel frequencyresponses are calculated by using the reference frequency spectrum, thepresence or the absence of the delay units 33 through 35 is negligiblein effect.

The channel frequency responses are calculated in the channel frequencyresponse calculating units 13 through 15 by using the OFDM receivedsignal frequency spectra output from the first transformers 7 through 9and the reference frequency spectrum output from the frequency spectrumselector 32 or the re-modulated frequency spectrum obtained byre-modulating the digital signal sequence by the re-modulator 31. Thecalculated channel frequency responses are respectively input into thefiltering units 23 through 25, which then eliminate distortions includedin the channel frequency responses and output the distortion-freechannel frequency responses.

The operation of the filtering units 23 through 25 is similar to thatperformed by the filtering unit 50 discussed with reference to FIGS. 11and 12. The OFDM received signal frequency spectra output from the firsttransformers 7 through 9 are input into the selector unit 19 as firstinput signals A, while the distortion-free channel frequency responsesoutput from the filtering units 23 through 25 are input into theselector unit 19 as second input signals B.

The selector unit 19 compares the OFDM received signal frequencyspectra, which serve as the first input signals A, generated in thebranches #1 through #n, and selects the branch that has generated theOFDM received signal frequency spectrum having the maximum amplitude orthe maximum power. The OFDM received signal frequency spectrum and thechannel frequency response which are output from the selected branch arethen input into the distortion compensator 22. The distortioncompensator 22 then compensates for the distortions of the OFDM receivedsignal frequency spectrum output from the selector unit 19 by using thechannel frequency response output from the selector unit 19, and outputsthe distortion-compensated OFDM received signal frequency spectrum.Subsequently, the demodulator 20 demodulates the frequency spectrum intoa digital signal sequence.

As noted above, in a system which transmits digital signal sequences byusing a slot containing a know data sequence at its head, the datasequences subsequent to the known data sequence are re-modulated in there-modulator 31, then input into the frequency spectrum selector. Thefrequency spectrum selector selectively outputs the reference frequencyspectrum or the re-modulated frequency spectrum to the channel frequencyresponse calculating units 13 through 15 of the respective branches #1through #n via the frequency spectrum selector 32. This processing isgenerally referred to as a “decision directed process” and is repeatedlyperformed. Namely, the channel frequency response is calculated from thefrequency spectrum of the digital signal sequence demodulated at a timek and from the OFDM signal frequency spectrum received at a time k. Thedistortions contained in the output of the first transformers at a timek+1 are compensated for, based on the calculated channel frequencyresponse.

According to the ninth embodiment, the updated channel frequencyresponses can be calculated in the respective branches #1 through #neven under a dynamic propagation environment, thereby enhancing thereceiving characteristics.

FIG. 14 is a diagram illustrating the bit error rate performance, i.e.,the receiving characteristics of the OFDM diversity receiving apparatusconstructed in accordance with the ninth embodiment of the presentinvention shown in FIG. 13. In this diagram, the horizontal axisrepresents the ratio of bit energy to noise energy Eb/No(dB), while thevertical axis indicates the bit error rate (BER). The bit error rateperformance has been obtained by using the QPSK modulation method undera 2-ray independent multipath propagation environment (fixed 2-raypropagation model). In the diagram, τ indicates a time differencebetween the appearances of the two rays (a desired path signal and anundesired path signal), and a D/U ratio represents a ratio of desiredsignal power to undesired signal power. In FIG. 14, the bit error rateperformance is shown when the D/U ratio is 0 (dB) and 5 (dB). FIG. 14reveals that regardless of the D/U ratio, the receiving characteristicsof the diversity receiving apparatus of the present invention can besignificantly improved over the known receiving apparatus withoutdiversity.

In this embodiment, assuming that a receiving apparatus is moving, there-modulator 31, the frequency spectrum selector 32, and the delay units33 through 35 are added to the eighth embodiment shown in FIG. 10.However, the above-described elements may be added to one of the firstthrough sixth embodiments.

As is seen from the foregoing detailed description, the presentinvention offers the following advantages. The diversity receivingoperation is performed by selecting from a plurality of diversitybranches the branch that has generated the OFDM received signalfrequency spectrum or the channel frequency response having the maximumpower or the maximum amplitude. It is thus possible to inhibit thedegradation of the receiving characteristics caused under a multiplereflection propagation environment.

What is claimed is:
 1. A receiving apparatus comprising: a plurality ofdiversity branches, each of said diversity branches including a receiverfor receiving an orthogonal frequency division multiplexing signal andoutputting an orthogonal frequency division multiplexing receivedsignal, a transformer for transforming the orthogonal frequency divisionmultiplexing received signal into a orthogonal frequency divisionmultiplexing received signal frequency spectrum and outputting theorthogonal frequency division multiplexing received signal frequencyspectrum, and a channel frequency response calculating unit forcalculating a channel frequency response in accordance with theorthogonal frequency division multiplexing received signal frequencyspectrum and a reference frequency spectrum; and a selector unit forselecting the diversity branch that has generated the channel frequencyresponse having a maximum amplitude or a maximum power.
 2. A receivingapparatus according to claim 1, further comprising: a re-modulator forre-modulating the digital signal sequence demodulated by saiddemodulator and for generating a re-modulated frequency spectrum; and afrequency spectrum selector for selectively outputting the re-modulatedfrequency spectrum or the reference frequency spectrum to said channelfrequency response calculating units.
 3. A receiving apparatus accordingto claim 1, wherein said selector unit comprises a plurality ofsynthesizers for synthesizing amplitudes or power of line spectraforming first input signals, and a comparator for comparing outputs ofsaid synthesizers, thereby performing a selecting operation based on acomparison result of said comparator.
 4. A receiving apparatus accordingto claim 1, wherein said selector unit comprises a comparator forcomparing, among line spectra forming first input signals, amplitude orpower of line spectra of the same frequency, thereby performing aselecting operation based on a comparison result of said comparator. 5.A receiving apparatus comprising: a plurality of receivers, each of saidreceivers for receiving an orthogonal frequency division multiplexingsignal and outputting an orthogonal frequency division multiplexingreceived signal; a plurality of transformers, each of said transformersfor transforming the orthogonal frequency division multiplexing receivedsignal from a corresponding one of the plurality of receivers into aorthogonal frequency division multiplexing received signal frequencyspectrum; at least one reference frequency spectrum generator forgenerating a reference frequency spectrum relative to the orthogonalfrequency division multiplexing received signal; a plurality of channelfrequency response calculating units, each of said channel frequencyresponse calculating units for calculating a channel frequency responsein accordance with the frequency spectrum output from a correspondingone of the plurality of transformers and the reference spectrum; aplurality of distortion compensators for compensating for a distortionof the frequency spectrum output from each of said transformers by usingthe corresponding channel frequency response; a selector unit forselecting, from among channel frequency responses from said channelfrequency response calculating units applied thereto as first inputsignals, the first input signals having a maximum amplitude or a maximumpower, and from among distortion compensated frequency spectra outputfrom said distortion compensators applied thereto as second inputsignals, the second input signals correspond to the selected first inputsignals; and a demodulator for demodulating an output signal of saidselector unit into a digital signal sequence.
 6. A receiving apparatusaccording to claim 5, further comprising a filtering unit disposedbetween each of said channel frequency response calculating units andsaid distortion compensators, for filtering the channel frequencyresponses calculated by each of said channel frequency responsecalculating units.
 7. A receiving apparatus according to claim 5,further comprising: a re-modulator for re-modulating the digital signalsequence demodulated by said demodulator and for generating are-modulated frequency spectrum; and a frequency spectrum selector forselectively outputting the re-modulated frequency spectrum or thereference frequency spectrum to said channel frequency responsecalculating units.
 8. A receiving apparatus according to claim 5,wherein said selector unit comprises a plurality of synthesizers forsynthesizing amplitudes or power of line spectra forming the first inputsignals, and a comparator for comparing outputs of said synthesizers,thereby performing a selecting operation based on a comparison result ofsaid comparator.
 9. A receiving apparatus according to claim 5, whereinsaid selector unit comprises a comparator for comparing, among linespectra forming the first input signals, amplitude or power of linespectra of the same frequency, thereby performing a selecting operationbased on a comparison result of said comparator.
 10. A method forreceiving a diversity signal for use in a radio communication systemthat uses an orthogonal frequency division multiplexing technique, saidmethod comprising the steps of: providing a plurality of diversitybranches, each of said diversity branches including a receiver forreceiving an orthogonal frequency division multiplexing signal via anantenna and outputting the orthogonal frequency division multiplexingreceived signal, a transformer for transforming the orthogonal frequencydivision multiplexing received signal into a frequency spectrum andoutputting the orthogonal frequency division multiplexing receivedsignal frequency spectrum, and a channel frequency response calculatingunit for calculating a channel frequency response in accordance with theorthogonal frequency division multiplexing received signal frequencyspectrum and a reference frequency spectrum; and selecting the diversitybranch that has generated the channel frequency response having amaximum amplitude or a maximum power.
 11. A method for receiving adiversity signal for use in a radio communication system that uses anorthogonal frequency division multiplexing technique, said methodcomprising the steps of: receiving an orthogonal frequency divisionmultiplexing signal via an antenna and outputting the orthogonalfrequency division multiplexing received signal; transforming theorthogonal frequency division multiplexing received signal into afrequency spectrum; generating a reference frequency spectrum relativeto the orthogonal frequency division multiplexing received signal;calculating channel frequency responses in accordance with the frequencyspectrum and the reference frequency spectrum; compensating fordistortion of the frequency spectrum by suing the corresponding channelfrequency response; selecting and outputting, in a case where channelfrequency responses are determined as first input signals and wheredistortion-compensated frequency spectra are determined as second inputsignals, wherein the second input signals correspond to the first inputsignals having a maximum amplitude and/or a maximum power; anddemodulating an output signal of said selecting step into a digitalsignal sequence.